GMC Repair: gmc 1 ton brougham mtr. home, perfect alignment, fan clutch

Question
I have the 5.7 ltr. eng. My problem is I keep throwing the water pump belt off. In all cases it actually breaks. Oh it's a 1984 GMC.iT ALSO TURNS THE ALTERNATOR.

oN MY LAST TRIP iT CAUSED THE ENGINE TO OVERHEAT AND i HAD TO REPLACE BOTH HEADS.

If I keep alow speed say below 50 the belt seems to stay put, over 50 to 65 or more and it trows the belt.

HELP PLEASE!!!!!!

Answer
Hi Morris. Without actually seeing the vehicle this will be difficult. First thing you will need to do is make absolutely certain that the pulleys are in perfect alignment, any variation and it will throw off the belt at higher rpm. Next, make sure there is no oil or any other liquids getting on the belt. The next thing is to make sure that the adjustment is right, too loose and it will also fall off, too tight and you will ruin the pump or the alternator.Make sure there is no side to side play in the waterpump,  and be certain none of the pulleys are warped or bent. Be certain that there is no drag on the pump or the alternator which would cause the belt to heat up and stretch and fall off. Check your fan clutch and make sure it turns freely when the engine is cool, if it is locked it will strain the belt and make a loud whooshing noise as you gain rpm. That's about all I can come up with for now, please let me know how you made out, i am very curious . MANFRED